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Thyroid Physiology - Coggle Diagram
Thyroid Physiology
Synthesis and Release of Thyroid Hormones
TSH Receptor
Present on thyroid cells, and has 7 transmembrane domains
Hormones
2 Main hormones: L-thyroxine T4, L-triiodothyronine T3
Iodine is the major substrate for these hormones
Production: Occurs in thyroid follicles: Consists of single layer of epithelial cells forming a spherical structure, external layer faces blood capillaries
Iodine Trapping
Thyroid and kidneys remove dietary iodine from gut
Thyroid cells concentrate iodine using a sodium/iodide symporter located on cell membrane surface
Synthesis
Tg has tyosine residues to form MIT and DIT
DIT + DIT -> T4, DIT MIT -> T3
Regulation
Iodine Availability: Availability regulates synthesis
Iodine Excess: Acutely inhibits hormone synthesis by blocking thyroglobulin iodination
TSH STimulates every step: Expression of NIS, TPO, Tg, and formation of T3 and T4
Deiodination
80% of T3 comes from deripheral dewiodination of T4 by type 1 deiodinase
Pathophysiology
Hypothyroidism
Condition characterized by exposure of tissues to subnormal amounts of thyroid hormone
Effects in:
Fetus + Neonate: Defective neuronal Development
Childhood: Growth and development
All ages: Metabolic activity, increased polysaccharide depositions
Findings
Enlarged tongue, Loss in last 3rd of eyebrows loss of relaxation of deep tendon reflexes
Growth and Development: Mental deficiency, delayed skeletal growth, delayed teeth
CNS: Intellecutal impairment, slow mentation, tiredness, depression, cerebellar ataxia, psychosis
Muslces: Weakness, cramps, delayed tendon reflexes
CV: Bradycardia, cardiomegtaly, pericaridal effuision, hypertension
Common Symptoms
Cold intolerance, skin is dry, puffy face, hair is coarse, enlarged tongue, hoarseness, constipation
Causes
Hashimoto's, Atrophic, Idodine/drug induced, Surgery, Radiation, Biosynthetic defects, pituitary disease
Hashimotos
Pathophysiology
Plasma cells and lymphocytic infiltration from autoimmune conditions
Tests: Positive thyroid antibodies (TPO)
Enzyme Defect leading to lack of production of thyroid hormone can cause boost of TSH in compensation
Investigations
TSH, Free T4, Free T3, antibodies (TRAb - Graves, TPO - Hashimotos)
Thyroid Scan: Radioisotope, and thyroid is scanned with counter to indicate function (hot, making hormone, cold - not making hormone)
Thryoid Ultrasound: Determine anatomy, size, detection and characterize nodules or cysts
Fine Need Aspiration: Determine cytopathology, diagnose thyroid cancer
TSH Assay: Most sensitive marker for hypothyroidsm - Will be elevated in low hormone levels
In pituitary and hypothalmic conditions, TSH is not elevated
Hyperthyroidism
Symptoms and Signs
Nervousness, Heat intolerance, insomnia, sweating, diarrhea, restlessness, warm,moist skin, tremor, diffuse goitre
Causes
Graves disease, Toxic Nodular goitre, hyperthyroid phase of subactue thyroiditis, excessive thyroid hormone
Thyrotoxicosis
Low TSH leve: Increase T3 and T4, increase RAIU if overproduction is cause, and elevated TRAb in graves disease
Graves Disease
TRAb antibody binds to TSH which activates the receptor to produce hormone
Manifestations
Hyperthyroidism, opthalmopathy, pretibial myxedema
Eye Changes: Periorbidal swelling, lid retraction and lid lag
Infiltrative ophthalmopathy: Proptosis, increase edema of lids and conjuctiva, chemosis
Thyroiditis
Inflammation of thyroid causing release of thyroid hormone that is self limiting
Phases: Hyperthyruoid, hypothyroid, recovery phase
NSAId or prednisone for pain, beta blocker for hyper thyropid symtpoms
Nodular Disease
Causes
Common: Cyst, colloid, benign neoplasm, papillary carcinoma
Uncommon: Granulomatous thyroiditis, infections, follicular, medullary, anaplastic carcinoma
Treatment
Toxic nodule: Anti-thyroid drugs, radioactive iodine, surgery
Nontoxic: Observation, Thyroixine suppresion, surgery
Action of Thyroid Hormone
T4 is converted to T3 (active hormone) and binds to receptor to regulate genes
Important for fetal development and childhood growth; deficiency leads to cretinism
Cretinism: Iodine deficiency, absence of goitre
Hypothalamic-Pituitary-Thyroid Axis
TRH from Hypothalamus Stimulates
TSH from Anterior Pituitary
Secretion of T4, T3 Which inhibits